1. Field of the Invention
The present invention relates to the fabrication of semiconductor integrated circuits and, in particular, to a self-aligned masking technique for use in ultra-high energy implants, i.e. implants of 1 MeV and greater. The technique has application to the formation of both localized buried implants and localized buried isolation structures.
2. Discussion of the Prior Art
The technology of integrated circuits is based upon controlling electric charge in the surface region of a semiconductor material. Typically, the semiconductor material is crystalline silicon. Control of electric charge in the crystalline silicon lattice is achieved by introducing impurity or "dopant" atoms into selected regions of the lattice.
The regions of the silicon lattice substrate to which dopant atoms are to be introduced are defined by transferring a corresponding pattern from a photographic "mask" to the substrate surface by a photolithographic, or "photomasking", process. In a typical sequence of steps in the photomasking process, a layer of silicon dioxide is first grown on the surface of the silicon substrate. A thin coating of photosensitive material, known as photoresist, is then formed on the oxide layer. The "negative" photoresist is then exposed to light through the mask. The portion of the photoresist not covered by opaque portions of the mask polymerize and harden as a result of this exposure (for a "positive" photoresist, the results would be the reverse). The unexposed portions are then washed away, leaving a photoresist pattern on the oxide surface that corresponds to the mask pattern. The portions of the silicon oxide that are not covered by the photoresist mask are then etched utilizing appropriate chemical procedures. The photoresist is then stripped, leaving an oxide layer that includes a desired pattern of "windows" through the oxide to the silicon surface. Dopant atoms are then introduced through the windows to the exposed silicon either by diffusion or by ion implantation.
Dopant diffusion is performed by placing the silicon substrate in a furnace through which flows an inert gas that contains the desired dopant atoms, causing the dopant atoms to diffuse into the exposed regions of the silicon surface.
In an ion implantation process, dopant atoms are introduced into the silicon by bombarding the exposed silicon regions with high-energy dopant ions. During the implantation process, the depth of penetration of the dopant ions into the silicon lattice is controlled by the ion implant energy, which is set by an accelerating field. The density of the implanted ions is controlled by the implant beam current. Typical commercial implant energy levels range from 30-200 kilo-electron-volts (KeV). Generally, a 1 micron layer of polysilicon, oxide or nitride is sufficient as a stopping material for these KeV implants.
When implanted dopant ions penetrate the silicon surface, they damage the lattice by producing defects and dislocations, in effect amorphizing the crystalline silicon structure. These localized amorphized regions are recrystallized by annealing the silicon at temperatures on the order of 500.degree.-600.degree. C. subsequent to the ion implantation step.
Recently, ultra-high energy implant machines that operate in the million-electron-volt (MeV) range have become commercially available. The ability to impart MeV range implant energies translates to the ability to create integrated circuit technologies that take advantage of the fact that dopant species can now be placed deeply into the silicon substrate at very high concentrations. However, full utilization of these ultra-high energy (i.e 1 MeV and greater) implants requires new techniques to create masks that can be used both as implant stoppers and to effectively pattern the silicon substrate target as desired.
The approach to masking MeV implants into silicon, for example, differs in kind from KeV implants. Differences arise due to the fact that relatively massive quantities of structurally firm material must be used to adequately stop the ultrahigh energy dopant ions from reaching the substrate other than in the desired regions defined by the mask. Moreover, materials used for ultra-high energy masking should possess qualities that permit differential etching for creation of special purpose implant structures.